Dogs and cats have abundant loose alveolar tissue and easily can accommodate large volumes of material in this subcutaneous space. The dorsal neck is seldom used for subcutaneous injections because the skin is somewhat more sensitive, causing some patients to move abruptly during administration. A wide surface area of skin and subcutaneous tissue over the dorsum from the shoulders to the lumbar region makes an ideal site for subcutaneous injections.
Administration of drugs, vaccines, and fluids by the subcutaneous route represents the most commonly used route of parenteral administration in dogs and cats. For small volumes (<2 mL total), such as vaccines, a 22- to 25-gauge needle generally is used. The site most often used is the wide area of skin over the shoulders. The large subcutaneous space and the relative lack of sensitivity of skin at this location make it an ideal injection site. Cleaning of the skin with alcohol or other disinfectant generally is performed before injection. Several injection techniques are used. A common technique entails grasping a fold of skin with two fingers and the thumb of one hand. Gently lift the skin upward. Using the opposite hand, place the needle, with syringe attached, through the skin at a point below the opposite thumb. Aspiration before injection is not typically necessary when using this route of administration. After administration and on removal of the needle from the skin, gently pinch the injection site and hold it for a few seconds to prevent backflow of medication or vaccine onto the skin.
When larger volumes are to be administered — fluids in dehydrated dogs and cats — the skin directly over the shoulders is the injection site most commonly selected. Generally, only isotonic fluids are administered by the subcutaneous route. Depending on the patient’s size, needles ranging from 16 to 22 gauge can be used. Because of the larger volumes of fluid involved, warming of the fluids before administration is recommended. Doing so can enhance significantly the patient’s tolerance for the displacement of skin during the period of administration and, in small patients, prevent hypothermia.
Depending on the rate of administration and breed of dog, relatively large volumes of fluid generally can be given in one location. Cats typically tolerate 10 to 20 mL/kg body mass in a single location. Large dogs can tolerate volumes greater than 200 mL of fluid in a single location. When administering large volumes, it is usually not necessary to use multiple injection sites for purposes of distributing the total fluid volume. Doing so actually may increase the risk of introducing cutaneous bacteria under the skin. Because the administration time required to deliver larger volumes is longer, and the injection needle will be placed in the skin for extended periods, it is appropriate to cleanse and rinse the skin carefully before actually inserting the needle. Isotonic, warmed fluids may be administered by large syringe or through an administration tube attached to a bag. Monitor skin tension and the patient’s comfort tolerance throughout the procedure.
Although fluid absorption begins almost immediately on subcutaneous administration of fluids, significant pressure caused by the bolus of fluid delivered can develop within the fluid pocket. On removal of the needle, firmly grasp the injection site with the thumb and forefinger for several seconds. The procedure is not complete until one has verified that back-leakage of fluid from the subcutaneous space onto the skin is not occurring. Depending on the patient’s hydration status and physical condition, fluid absorption may take from 6 to 8 hours.
Note: Not all parenteral medications can be administered safely by the subcutaneous route. When administering any compound by the subcutaneous route, verify that the product to be administered is approved for subcutaneous administration. Serious reactions, including abscess formation and tissue necrosis, can occur.
The rate of absorption of fluid administered by the subcutaneous route largely depends on the patient’s hydration state and vascular and cardiac integrity. For that reason, the subcutaneous route is not recommended to manage patients in hypovolemic shock. Exceptions to this do exist — for example, when in a life-or-death situation access to a vein is simply not possible. Subcutaneous or intraosseous (see the following discussion) fluid administration may be the only option available.
Implanted Subcutaneous Fluid Ports
Recently, implantable subcutaneous ports (GIF-Tube Single Implant Kit for Subcutaneous Fluid Administration, various models available. Phoenix, Arizona, www.practivet.com) have been introduced for use in patients requiring regular administration of subcutaneous fluids at home. A 9-inch silicon tube is pre-placed under the skin and is sutured in place by a veterinarian. Objectively, this offers easy access to the subcutaneous space without the need for needle penetration. Owners simply attach a syringe or extension tube tip to the port and administer the appropriate volume of fluids at an appropriate rate and frequency.
Because of the usual requirement for long-term placement of an implantable fluid administration tube, there is some risk of infection under the skin and around the incision site. Some cats do not tolerate the device.